Spatially Enabling the Web with OGDI
Kenn Gardels, UC Berkeley
The use of geographic information is extending far beyond its traditional boundaries of mapping programs, to embrace a broader community of analysts, decision-makers, and interested citizens. With this has come increased interest in locating and using geodata available throughout the network, whether this is the entirety of a global spatial data infrastructure or in the confines of a corporate intranet. These data are of coursed maintained in a diversity of systems/organizations, presenting vast challenges for data access and interpretation. Geographic information system interoperability across platforms and data formats has proceeded in a series of initiatives addressing conversion tools, standardized interchange formats, application interfaces, and clearinghouses.Two major forces are at work in forging new demands on technologies for access to information: recognition by system builders and users that applications require complete, consistent, timely information; and expectations in the age of the World Wide Web that information should be easily found and used. While earlier efforts in the field of geographic information systems targeted mechanisms for inter-system data exchange, current efforts in the GIS industry, as exemplied by work by Open GIS Consortium participants, have redirected interest into specifications for geodata and geoprocessing interoperability within distributed computing environments. At the same time, software developers are attempting to take advantage of the proliferation of the Web as a basis for tools for making information available to Web users from specialized servers.
Although the proposed implementations fitting the Open GIS Abstract Specification are based on distributed computing platforms - CORBA, OLE/DCOM, and ODBC - they do not directly solve the problem of general Web-based access to a network of heterogeneous geodata. The Open Geographic Datastore Interface (OGDI) was written as a means of rendering geodata heterogeneity invisible to Web clients. It does so by defining a set of standard interfaces for connecting to datastores, describing a dataset’s organization and structure, extracting geodata objects, and establishing common regions of interest, projections, and coordinate systems. A key component of OGDI is gltp, a stateful network protocol for linking geodata servers and clients. It provides a mechanism for locating a remote data source, specifying its format, and defining the pathname or identification for a dataset. Specifically, it links client requests or queries to a software driver on the appropriate geodata server, and returns results from the server to the client application. The driver interprets OGDI queries and initiates the corresponding data retrieval operation on the native data store. The results are placed in a generic structure that models point, line, polygon, annotation, and raster information, which the OGDI-aware client can then use directly.
Under development now are assembler components between OGDI clients and OGIS servers. Although the OGDI and OGIS data models are not presently congruent, they are very similar in their ways of describing geodata - OGDI type families and OGIS feature types describe the same basic entity types. The assemblers act as proxies to make collections with exposed OpenGIS interfaces available via the gltp protocol for map visualization and modeling by relatively thin clients. Such clients can be standalone applications, downloadable applets, or plug-ins. At a higher level of abstraction, a map view may be rendered via standard html/http mechanisms to naive browsers.
